Carbonaceous materials are widely used in electrical storage cells, also referred to as “batteries” due to their efficiency and reasonable cost. Various forms of carbonaceous materials are used. One such carbonaceous material is graphite, which is known to be useful in rechargeable storage cells, also referred to as “rechargeable batteries”. In a salient example, graphitic materials are known to be useful as anode materials in rechargeable lithium ion, “Li-ion” storage cells. Li-ion cells are mainly used as the power sources in portable electronic devices.
As opposed to other classes of rechargeable batteries, i.e., e.g., nickel-cadmium and nickel-metal hydride storage cells, Li-ion cells are increasingly popular due to their relatively higher storage capacity, and their easily rechargeable nature. Due to such higher storage capacity per unit mass or unit volume, Li-ion cells may be produced which meet specific storage and current delivery requirements as they are smaller than similarly rated, nickel-cadmium and nickel-metal hydride storage cells. Consequently, Li-ion cells are popularly used in a growing number of devices, i.e., digital cameras, digital video recorders, computers, etc., where small sized devices are particularly desirable from a utility or consumer standpoint. Nonetheless, rechargeable Li-ion storage cells are not without their shortcomings, certain of which are dependent upon their materials of construction.
Lithium manganese oxide powders, particularly lithium manganese oxide (LMO) spinel with stoichiometric formula LiMn2O4 powders have been extensively investigated as the cathode material for lithium ion batteries because the material is relatively easy to produce, is inexpensive, environmentally benign, and relatively safe compared to other cathode materials such as lithium cobalt oxide. More importantly, lithium manganese spinel material has a high power capability, which is particularly suitable for energy conversion and storage and high power applications such as hybrid electrical vehicles. However, it has been found that this material exhibits a poor cycle life, particularly at an elevated temperature. Such a poor cycle life has been attributed to the dissolution of LMO in electrolyte because dissolved manganese ions migrate through the separator and deposit on the anode, causing degradation of the anode and cycle life.
Extensive research and development efforts have been spent on solving this problem; the resulting effective methods can be classified into two types: a) modification of the LMO spinel material to reduce the solubility and b) modification of the anode materials to prevent dissolved manganese species from depositing on the anode. Various chemical and physical methods have been used to modify the spinel materials, some of the effective methods include i) doping various metal and non-metal species such as cation species of magnesium, aluminum, and nickel etc. and anion species such as fluoride (F−), ii) coating inorganic salts such as AlF3 and ZrO etc., iii) changing the crystalline structure of the spinel etc. However, it has been found that these effective methods cause a reduction in the reversible capacity in exchange for an improvement of cycle life. The net effect of doping various species is preventing the valence or oxidation state of Mn from being reduced to 3.5+. Use of alternative anode materials such as lithium titanium oxides and amorphous or hard carbons has been reported by many researchers.
It is reported that amorphous carbon or hard carbon powders are currently used as the anode material in the lithium ion cell with modified LMO spinel powder as the cathode material. Because these amorphous or hard carbon powders exhibit a low coulombic efficiency on the initial charge, a thick ionic conductive but electronic insulating layer is built on the carbon surface during the initial formation; such an electronically insulating layer may prevent manganese ions from depositing on carbon particle surface. In addition, the low coulombic efficiency of the anode materials also prevents the cathode from being discharged completely.
However, use of amorphous or hard carbon as the anode material would severely limit the energy density of the lithium ion cells because these materials have several shortcomings, namely i) low density (typically less than 1.8 g/cc), ii) low useable specific capacity (less than 250 mAh/g at an electrode potential below 1.0 volts versus Li), iii) relatively high electrode potential versus Li (this results in a lower cell voltage), and iv) very low coulombic efficiency (less than 75% within a cell). The other known alternative anode materials such as lithium titanium oxide (LTO) spinel with a general formula Li4Ti5O12 also have shortcomings similar to those of hard carbons. For example, the specific capacity of LTO spinel is about 150 mAh/g and the average potential is 1.5 volts versus Li, compared to about 0.2 volts versus Li for graphite electrode. Furthermore, both hard carbon and LTO materials are fairly expensive.
There is a need for better compatible anode materials for the lithium ion batteries with LMO spinel cathode.